This invention relates to the a system for accurate thermal compensation for the recorded density of thermal printers which perform multi-tone image printing, and it is widely applicable to thermal transfer printers or the like used as hardcopy devices for printing a television picture.
The thermal recording system which performs thermal recording by using a thermal transfer ink film or the like can more readily deal with colors and can be made more compact than ink-jet system an electronic photographic system, and because of its further advantages in picture quality, cost, maintenance, etc., this system is widely adopted for hardcopy devices which record pictorial images.
Generally, a color printer based on the thermal transfer system uses a thermal head, which comprises a lateral alignment of heating elements and an inked ribbon on which three colors of yellow (Y), magenta (M) and cyan (C) are distributed, and operates on the basis of three-color face sequential recording in which the recording paper is repositioned in each turn of color. For recording a pictorial image such as a television signal, the sublimation dye type thermal transfer printing is superior because of its higher performance in both remelt and toning, the controllability of recorded density and the smoothness of tonal recording, as compared with the system of dizzer, density pattern, etc.
However, such a system as the sublimation dye thermal transfer printing, which performs analog tonal density recording by varying the applied energy based on the current pulse width modulation, has its recording density dependent on the environmental temperature and is susceptible to the cumulative heat of the thermal head, and therefore it is difficult to have a stable production of recorded density. This temperature dependency is a major restricting limiting factor against the enhancement of the picture quality in developing these printers.
In the case of full color recording on a face sequential basis, the difference in environmental temperature and the difference of cumulative heat among colors result in a broken balance of the density of colors and in the variation of hue, and therefore more strict thermal compensation is required.
To cope with these problems, there have been proposed (1) a method of controlling the pixel applied energy with reference to the temperature of the head mount detected with a temperature detection means and the time length which has expired since the previous driving of the heating elements counted with a time count means (as disclosed in Japanese Patent Unexamined Publication No. 59-127782), (2) a method of controlling the applied energy by providing several ROM tables, in which relationships between the tonal level and current pulse width for several environmental temperatures are stored, and selecting a ROM in response to the temperature of the head mount or the like (as disclosed in Japanese Patent Unexamined Publication No. 58-164368), and (3) a method of controlling the pixel applied energy with reference to the amount of cumulative heat calculated from the states of several lines of heating elements which have been activated in the past and of adjoining elements (as disclosed in Japanese Patent Unexamined Publication No. 59-127781). These methods, however, involve the following deficiencies.
Thin-film thermal heads or the like used generally have a structure as shown in FIG. 2. The head involves a first dominant heat accumulation in the head mount determined from the thermal capacity of the head mount and its heat dispersing resistance to the atmosphere, a second heat accumulation in the heating element substrate, and a third heat accumulation in the heating elements themselves, and they have distinct thermal time constants of the order of several minutes, several seconds and several milliseconds, respectively.
The thermal compensation for two-level recording, which is mainly aimed at the stable reproduction of clear dot printing without the influence of the environmental temperature and the heat accumulation of the head at a high printing speed, merely needs a rough compensation accuracy, although the third heat accumulation in each heating element of pixel needs to be compensated.
In contrast, the thermal compensation for tonal recording has its density compensation accuracy raised to the grade of tone steps, thereby fulfilling the requirement of the accurate production of tone in steps through the recordings at arbitrary environmental temperatures. Because of its tighter requirement of the picture quality than of the recording speed, this recording system is less affected by the third heat accumulation in the heating elements themselves, although it needs accurate thermal compensations for the second heat accumulation in the heating element substrate and the first heat accumulation in the head mount.
The technique described in the above patent publication 59-127782 bases the compensating operation on the prediction of the third heat accumulation in pixel-wise heating elements from the time expiration since the previous recording action with the intention of high-speed two-level recording, and therefore it cannot be applied to the thermal compensation for the tonal recording.
The technique described in the above patent publication 59-127781 is intended to evaluate the subsequent applied energy by calculating the third heat accumulation in the heating elements themselves from weighted summation of energy-application patterns of specific heating elements used for the past several lines and a joining elements. This simple and more experimental, rather than theoretical, method for the calculation of the heat accumulation state can be useful for two-level recording, whereas it cannot compensate accurately for all tone steps, or it can even disturb tone levels, in tonal recording.
The technique described in the above patent publication 59-127782 based the applied energy control on the switching of ROM tables, in which relationships between the tonal level and current pulse width at several environmental temperatures are set, in response to such environmental temperature as the head mount temperature. Although this technique is intended for tonal recording, the control solely relies on the head mount temperature which can be measured during the recording operation, and it not only suffers from a significant delay of detection, but frequently fails to correlate the detected temperature with the recorded density for some object of recording. Therefore it is incapable of performing a sufficient density compensation.
None of the foregoing prior art techniques considers the second heat accumulation in the heating element substrate, failing in the density compensation against a great variation of cumulative heat of the order of several seconds in time, and it does not accomplish a sufficient thermal compensation for the tonal recording. Moreover, even in the case of successful prediction of the second heat accumulation in the heating element substrate, which measurement is difficult in reality, the formulation of applied energy based on the amount of cumulative heat is not yet established, and values of compensation parameter for each temperature are solely derived from experimental data and simulation data. These values are only significant for specific recording conditions and values for other conditions need to be determined on an experimential or trial-and-error basis, and therefore these methods involve an extremely difficult problem in achieving a correct production of recorded density at all tone levels.